1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens. More specifically, the present invention relates to a zoom lens useful as a photographic optical system used in an image pickup apparatus.
2. Description of the Related Art
A zoom lens that uses a convex lens group at the front is known as a positive-lead type zoom lens. As a positive-lead type zoom lens, a four-unit zoom lens has been used. A four-unit zoom lens of the positive-lead type generally includes, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power.
U.S. Pat. No. 7,382,549 discusses a small-size zoom lens whose first lens unit is constituted by one negative lens and one positive lens and whose second lens unit is constituted by two negative lenses and one positive lens. U.S. Pat. No. 7,206,137 discusses a small-size zoom lens whose first lens unit is monotonously moved towards the object side during zooming. U.S. Pat. No. 7,304,805 discusses a five-unit zoom lens which includes, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power.
In order to achieve a small-size positive-lead type zoom lens having a predetermined level of zoom ratio, it is necessary to increase the refractive power of each lens unit included in the zoom lens with as small a number of lenses as possible. However, the positive-lead type zoom lens necessarily requires a thick lens as the refractive power of each lens surface becomes large.
As a result, if the total size of the entire zoom lens is not sufficiently reduced, various types of aberrations often occur. Based on this, positive-lead type zoom lenses having small size, wide angle of view, and a high zoom ratio have been proposed. However, in an image pickup apparatus (camera) in which each lens unit is retractable when the camera is not used or powered off, a conspicuous error, such as a tilt of a lens or a lens unit, may occur due to its structural characteristic.
In this case, if a lens or a lens unit has a high sensitivity to errors, the optical performance may greatly degrade. For example, the phenomenon of image shake may occur during zooming. In order to prevent the above-described problems, it is desired that the sensitivity of a lens or a lens unit be suppressed to a level as low as possible without affecting performance.
In a four-unit zoom lens or a five-unit zoom lens, in order to achieve a small-size zoom lens having a high zoom ratio and a high optical performance at the same time, it is desirable to appropriately set size and optical parameters for each constituent lens included in the second lens unit and the third lens unit among the lens units included in the zoom lens. More specifically, when a small-size zoom lens with high optical performance is desired, it becomes significant to appropriately set a lens configuration, such as the zoom type (the number of lens units and the refractive power of each lens unit), the moving locus of each lens unit during zooming, and the share of power on each lens unit for variable magnification.
One of the main obstacles in obtaining the above-desired goals is that the total size of the zoom lens becomes large when the zoom ratio is increased. In addition, in a large size zoom lens, the amount of variation of various aberrations that occur during zooming may increase. As a result, it becomes very difficult to achieve high optical performance for the entire zooming range and for the entire image plane.